Correct spelling, explanation: incumbent is a rather formal English word, which derives from the Latin form incumbens and was first used to mean lying on. Later, however, the word got the meaning of holding a position. When incumbent entered English it initially referred to a person who held a church office, but later it started to be used to refer to a person holding any office.

Definition of incumbent:
1. adjective – when someone officially has the named position,
The incumbent mayor of the city has to deal with a number of problems.
2. noun – a person that holds some position in a company, industry, etc.,
The incumbent was given an award for his creative projects.

Collocations with incumbent:
A few common collocations with incumbent are:
1. be incumbent on someone – when something is necessary for someone
He felt it incumbent on his position in the company to start the subject of his promotion.
2. incumbent president – the president at the present time
The meeting with the incumbent president was canceled due to his illness.
3. it is incumbent on someone to do something – means that someone has a duty to do something
It is incumbent on you to report to your boss at the end of every week.

Incorrect spelling, explanation: the word incumbent causes some spelling problems as both – the learners of English and its native speakers – often make a mistake and spell the word with the -a letter instead of -e letter. It results in the form incumbant, which is wrong. The mistake probably results from the pronunciation, which doesn’t clearly suggest the right spelling.